A technique is known, in which a mat composed of an insulator is provided between a catalyst carrier which generates the heat by applying the electricity and a case which accommodates the catalyst carrier (see, for example, Patent Document 1). According to this mat, it is possible to suppress the electricity from flowing through the case when the electricity is applied to the catalyst carrier. However, water contained in an exhaust gas is sometimes condensed on a wall surface of an exhaust tube, for example, immediately after the startup of an internal combustion engine. The water, which is converted into the liquid, flows to the downstream side while being pushed by the exhaust gas, and the water arrives at the catalyst. If the water in the liquid state enters the mat, and the water arrives at an electrode, then the insulation resistance between the electrode and the case is lowered, and hence it is feared that the electricity may flow from the electrode to the case.
In order to suppress the water from passing through the mat and arriving at the catalyst carrier, the mat is divided by an inner tube in some cases. However, the heat from the catalyst and the heat of the exhaust gas are hardly transferred to the outside beyond the inner tube. Therefore, the temperature of the mat existing outside the inner tube is hardly raised. Further, the heat is released to the outside of the case from the mat existing outside the inner tube. Therefore, the temperature of the mat existing outside the inner tube is easily lowered. Therefore, the water, which enters the mat disposed outside the inner tube, tends to stay in the mat without being evaporated. The insulation resistance value between the electrode and the case is lowered by the water allowed to stay in the mat. Therefore, it is feared that the electricity may flow from the electrode to the case.